In the mouse thymus, a novel T cell subset, NK1.1+ T lymphocytes, has recently been recognized that expresses markers of the natural killer lineage and also has the unique potential to secrete large amounts of the lymphokine lL-4 upon primary stimulation, a property that makes it a candidate to influence the T helper phenotype (Th1 or Th2) of immune responses in which it is engaged. Its ligand appears to be encoded by members of the CD1 gene family, a set of MHC class 1-like molecules that are conserved in several mammalian species, including humans. Its T cell receptor (TCR) repertoire is highly restricted, using a single invariant TCR alpha chain, which is homologous to that used by a similar subset in humans. This conservation through speciation of genes encoding a set of TCRs and their ligand(s) demonstrates the biological importance of this T cell lineage. Our long-term objectives are to understand the rules governing the development and the activation of this novel subset and to determine their specific biological role in different species. The specific aims of this project are to: A- identify the nature of the CD1 ligands of NK1.1+ T cells: the products of two mouse genes, CD1.1 and CD1.2, can be recognized by NK1.1 + T cells, apparently in the absence of foreign antigens. Specific monoclonal antibodies will be generated. The differential recognition of CD1.1 and CD1.2 by subsets of NK1.1+ T cells, their modification by genes outside the CD1 region, and their different patterns of expression, will be investigated. B- determine the conditions of CD1 presentation during thymic selection: in the thymus, CD1 genes seem to be only expressed on immature thymocytes, suggesting that the unusual phenotype of NK1.1 + T cells may be related to the CD1-presenting cell-type. Transgenic mice will be generated where this expression is re-targeted to different cell-types in order to test the role of various ligand-expressing cells in thymic positive and negative selection as well as in the differentiation of this T cell lineage. C- test the role of CD1 in influencing the Th1/Th2 phenotype of immune responses: CD1-expressing transgenic cells will be used to recruit NK1.1 + T cells to the site of normal immune responses in order to formally test the hypothesis that the NK1.1+ T cells influence the Th1/Th2 phenotype of immune responses.